Orthogonal frequency-division multiplexing (OFDM) is a widely used digital modulation/multiplexing technique. Coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is being considered as a promising technology for future high-speed (e.g., 100+-Gb/s per-channel data rate) optical transport systems. In coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems, fiber nonlinearity imposes a severe limitation on optical transmission performance. For example, the quality of a single-polarization CO-OFDM channel can be severely degraded by self-phase modulation (SPM) when fiber dispersion is neglected or when the transmission link is inline-dispersion-compensated.
Self-phase modulation (SPM) is a nonlinear optical effect of light-matter interaction. An ultrashort pulse of light, when travelling in a medium, will induce a varying refractive index of the medium due to the optical Kerr effect. This variation in refractive index will produce a phase shift in the pulse, leading to a change of the pulse's frequency spectrum.
It has been proposed to improve SPM tolerance for single-polarization CO-OFDM by using digital nonlinearity compensation (DNLC) at the receiver and the transmitter to impose a “negative” phase shift on each OFDM signal sample proportional to its power via a single complex multiplication. It also has been shown that in single-polarization CO-OFDM transmission without dispersion, the SPM effect can be compensated for by applying in the time-domain a phase modulation Φcomp(t) that is proportional to the signal power P(t) according to Φcomp(t)=−γP(t)sLeff, where γ is the fiber nonlinear coefficient in units of W−km−1, s is the number of the fiber spans, and Leff is the effective fiber length of each span in units of km.
Polarization-division multiplexed (PDM) OFDM has been suggested for high spectral-efficiency 100+-Gb/s transmission. In PDM-OFDM, there are two orthogonal polarization components for each signal. However, existing methods for improving SPM nonlinear tolerance for a PDM-OFDM signal are lacking.